Tentar:
CREATE OR REPLACE VIEW my_view AS
WITH date_bookends AS (
SELECT LEAST(MIN(t.sampled_on), MIN(t.received_on), MIN(t.completed_on), MIN(t.authorized_on)) 'min_date'
GREATEST(MAX(t.sampled_on), MAX(t.received_on), MAX(t.completed_on), MAX(t.authorized_on)) 'max_date'
FROM SAMPLE t),
all_hours AS (
SELECT t.min_date + numtodsinterval(LEVEL - 1,'hour') date_by_hour
FROM date_bookends t
CONNECT BY LEVEL <= ( t.max_date - t.min_date + 1) * 24)
SELECT h.date_by_hour,
COUNT(CASE WHEN h.hour = TRUNC(s.sampled_on,'hh24') THEN 1 END) sampled#
COUNT(CASE WHEN h.hour = TRUNC(s.received_on,'hh24') THEN 1 END) received#
COUNT(CASE WHEN h.hour = TRUNC(s.completed_on,'hh24') THEN 1 END) completed#
COUNT(CASE WHEN h.hour = TRUNC(s.authorized_on,'hh24') THEN 1 END) authorized#
FROM all_hours h
CROSS JOIN sample s
GROUP BY h.hour
Sem usar fatoração de subconsulta:
CREATE OR REPLACE VIEW my_view AS
SELECT h.date_by_hour,
COUNT(CASE WHEN h.hour = TRUNC(s.sampled_on,'hh24') THEN 1 END) sampled#
COUNT(CASE WHEN h.hour = TRUNC(s.received_on,'hh24') THEN 1 END) received#
COUNT(CASE WHEN h.hour = TRUNC(s.completed_on,'hh24') THEN 1 END) completed#
COUNT(CASE WHEN h.hour = TRUNC(s.authorized_on,'hh24') THEN 1 END) authorized#
FROM (SELECT t.min_date + numtodsinterval(LEVEL - 1,'hour') date_by_hour
FROM (SELECT LEAST(MIN(t.sampled_on), MIN(t.received_on), MIN(t.completed_on), MIN(t.authorized_on)) 'min_date'
GREATEST(MAX(t.sampled_on), MAX(t.received_on), MAX(t.completed_on), MAX(t.authorized_on)) 'max_date'
FROM SAMPLE t) t
CONNECT BY LEVEL <= ( t.max_date - t.min_date + 1) * 24) h
CROSS JOIN sample s
GROUP BY h.hour
A consulta acessa a tabela SAMPLES duas vezes - a primeira vez para obter a data mais antiga e mais recente para enquadrar a construção do
date_by_hour valor. 
